In an article by Williams, et al., Fuel, December, 1990, Vol. 69, pp. 1474-1482, it was reported that the disposal of scrap tires is an increasing environmental problem. For example, estimates for the generation of scrap tires are 1.5.times.10.sup.6 tons per year in the European Community, 2.5.times.10.sup.6 tons per year in North America and 0.5.times.10.sup.6 tons per year in Japan. [Roy, et al., "Pyrolysis and Gasification", Ferrero, et al., Eds., Elsevier Applied Science, London, UK, 1989] The majority of this tire waste is dumped in open or landfill sites. However, tires do not degrade in landfills and open dumping may result in accidental fires with high pollution emissions. In addition, this method of disposal ignores the large energy potential of scrap tires. Incineration has been considered as an alternative to dumping in an effort to utilize the high calorific value of scrap tires (.apprxeq.36-40 Mj kg.sup.-1), but this disposal route may not maximize the potential economic recovery of energy and chemical materials from the waste. Pyrolysis of tires to produce liquid hydrocarbons and gases is currently receiving renewed attention [Roy, et al., supra; Williams, et al., "Pyrolysis and Gasification", Ferrero, et al., Eds., Elsevier Applied Science, London, UK, 1989; Cypres, et al. "Pyrolysis and Gasification", Ferrero, et al., Eds., Elsevier Applied Science, London, UK, 1989; Kaminsky, et al., "Thermal conversion of Solid Wastes and Biomass", Jones, et al., Eds., American Chemical Society Symposium Series 130, 1980; Wilkins, et al., J. Environ. Sci. Health, A18(6), 747, 1983; Kawakami, et al., "Thermal Conversion of Solid Wastes and Biomass", Jones, et al., Eds., American Chemical Society Symposium Series 130, 1980] since the derived products are easily handled, stored and transported and hence do not have to be used at or near the recycling plant. The derived oils may be used directly as fuel or added to petroleum refinery feedstocks. The oils may also be an important source of refined chemicals, since it has been shown that they contain high concentrations of potentially valuable chemical feedstocks, for example, benzene, toluene and xylene [Roy, et al., supra; Kaminsky, et al., supra; Collin, G., "Thermal Conversion of Solid Wastes and Biomass", Jones, et al., Eds., American Chemical Society Symposium 130, 1980]. The derived gases are also useful as fuel and the solid char may be used either as smokeless fuel, carbon black or activated carbon [Roy, et al., supra; Cypres, et al., supra; Kawakami, et al., supra].
Tires contain vulcanized rubber in addition to the rubberized fabric with reinforcing textile cords, steel or fabric belts and steel-wire reinforcing beads [Dodds, et al., "Scrap Tyres: a Resource and Technology Evaluation of Tyre Pyrolysis and Other Selected Alternative Technologies", U.S. Dept. of Energy Report, EGG-2241, 1983]. The most commonly used tire rubber is styrene-butadiene-copolymer (SBR) containing about 25 weight percent styrene. Other rubbers used in tire manufacture include natural rubber (cis-polyisoprene), synthetic cis-polyisoprene and cis-polybutadiene. The carbo black is used to strengthen the rubber and aid abrasion resistance, and the extender oil is a mixture of aromatic hydrocarbons which serves to soften the rubber and improve workability. Sulfur is used to cross link the polymer chains within the rubber and also hardens and prevents excessive deformation at elevated temperatures. The accelerator is typically an organo-sulfur compound which acts as a catalyst for the vulcanization process. The zinc oxide and stearic acid also act to control the vulcanization process and in addition enhance the physical properties of the rubber.
A number of commercial and pilot plant systems have been reported for the pyrolysis of automotive tire waste, for example externally heated rotary kilns, fluidized beds, continuous and batch fed static reactors and molten salt pyrolysis. The advantage of the pyrolytic treatment of scrap tires may be significantly enhanced if the process conditions can be used to optimize the final product composition and yield for the required end use. Kaminsky, et al., supra; have shown, using a fluidized bed pyrolyzer for scrap tires, that increasing the temperature from 640.degree. C. to 840.degree. C. produces an increase in the yield of carbon black, hydrogen, methane and benzene, and a decrease in the yield of oil. Kawakami, et al., supra, used a rotary kiln pyrolyzer and similarly showed a decrease in oil and increase in gas yield on raising the pyrolsysis temperature from 540.degree. C. to 740.degree. C. They also showed that the properties of the char in relation to carbon black were significantly altered over the temperature range. The Roy, et al. reference, above, used a vacuum pyrolysis reactor and showed a decrease in carbon black and increase in oil and gas yield on raising the temperature to 500.degree. C. The gas was mainly composed of hydrogen, carbon monoxide and carbon dioxide and hydrocarbons. Douglas, et al., "Symposium on Treatment and Recycling of Solid Wastes", Institute of Solid Wastes Management, Manchester, UK, 1974], using a fixed bed reactor, showed that increasing the heating rate within the reactor up to 45.degree. C./ min.sup.-1 produced an increase in the char and gas and decrease in the oil yields. They also showed that the gas composition was affected by the heating rate. Although there are some data on total oil, gas and char yields in relation to the thermal processing conditions, there are less data on the chemical composition of the products.
Polymeric materials, referred to hereinafter by the generic term "plastics", account for about 7% of municipal solid waste and up to about 20% of the waste by volume. This amounts to about 10 to about 12 million tons per year in the United States. Although plastics recycling is increasing, reprocessing and recycling generally requires segregation by type of plastic. Consumers, in general, and reprocessors often have no idea as to the composition of individual plastic articles. Consequently, processes for utilization of mixed plastic waste, particularly polystyrene, polypropylene and polyethylene, are urgently needed. The present invention provides a process for conversion of mixed plastic waste materials in combination with scrap automotive tires to a high quality synthetic crude oil which can be separated by fractionation into gasoline, diesel fuel and gas-oil components suitable as a feedstock to a catalytic cracker after removal of any sulfur contributed by the automotive tires. As used herein, the term "plastic waste" includes all forms of polymeric materials which require or will benefit from recycling, including processing scrap, municipal waste and recovered or recycled polymeric materials.
U.S. Pat. No. 4,724,068 to Stapp describes a process for hydrotreating hydrocarbon-containing feed streams, especially heavy oils. The process of the Stapp patent utilizes a polymeric treating agent for upgrading the composition of heavy oils. In accordance with the process, an upgrading process is provided comprising the step of contacting (a) a substantially liquid hydrocarbon-containing feed stream substantially simultaneously with (b) free hydrogen, (c) hydrogen sulfide and (d) at least one polymer selected from the group consisting of homopolymers and copolymers of olefinic monomers, in the substantial absence of a solid, inorganic cracking catalyst and a solid inorganic hydroconversion catalyst. The process is performed under conditions so as to obtain a product stream having higher API.sub.60 gravity and having a lower content of hydrocarbons boiling above 1000.degree. F. than the feed stream.
In accordance with the process of the Stapp patent, impurities contained in the hydrocarbon-containing feed stream are at least partially converted to a "sludge", i.e., a precipitate of metals and coke, which is dispersed in the liquid portion of the hydrocarbon-containing product stream. The sludge and the dispersed olefin polymers are then separated from the liquid portion of the hydrocarbon-containing product stream by any suitable separation means, such as distillation, filtration, centrifugation or settling and subsequent draining of the liquid phase. The hydrocarbon-containing product stream has an increased API.sub.60 gravity and lower content of heavy fractions. The weight ratio of olefin polymer to hydrocarbon-containing feed is described as being generally in the range of from about 0.01:1 to about 5:1, preferably from about 0.02:1 to about 1:1 and more preferably from about 0.05:1 to about 0.5:1. The Stapp patent generally describes a procedure for hydrovisbreaking a heavy oil with a mixture of hydrogen and hydrogen sulfide in the presence of olefin polymers followed by recovery of an improved hydrocarbon oil product after separation from the olefin polymers.